# Question #763a2

Aug 20, 2017

${M}^{2 +} H P {O}_{4}^{2 -}$ OR ${M}_{3} {\left(P {O}_{4}\right)}_{2}$

#### Explanation:

Oxides are salts of the ${O}^{2 -}$ ion.... Since these salts are necessarily neutral, we add the appropriate number of metal cations to give a NEUTRAL charge.....

So for $M O$ we had a salt of ${M}^{2 +}$; this would form a salt with phosphoric acid of the form....

${M}^{2 +} {\left(H P {O}_{4}\right)}^{2 -}$.....usually we encounter biphosphate salts in aqueous solution. However, the answer they are probably looking for is ${M}_{3} {\left(P {O}_{4}\right)}_{2}$ (these salts are actually quite rare!).

So can you give the formula for lithium, calcium, and iron(II), and iron(III) phosphates?